In aircrafts, detector means of the radar or laser telemeter type or of the stereoscopic imaging system type are used.
On the basis of range images provided by detector means delivering individual echoes or “plots”, it is known to obtain a terrain elevation database for the zone observed by the detector means. The database contains all of the relief of the terrain together with its obstacles. Nevertheless, the databases in the raw state also contains errors due to false echoes and these need to be eliminated or corrected.
This applies particularly in the field of photogrammetry. In photogrammetry, the terrain data is generally acquired by using lidar type scanning sensors (where lidar is short for light detection and ranging) mounted on board aircraft that are also provided with inertial units in order to obtain ground references with the best possible accuracy. In that technique, it is known to filter out false echoes that arrive during detection, preferably by eliminating them, since such echoes disturb measurements of the ground and of objects present thereon.
These echoes may be filtered by a morphological filter. Under such circumstances, on the basis of the acquired data, the images of the surface sheet are encoded on a gray scale and a point is given a value of a density that varies depending on its altitude. By implementing the mathematical morphology operators of erosion and of expansion, false echoes can be eliminated. Such filtering may also be optimized by implementing a progressive morphological filter that varies over time relative to the nature of the terrain.
Other filter means may also be implemented for photogrammetry, e.g. such as linear prediction using the least squares method in iterative manner over the entire surface. This smoothes the terrain model with elevations being averaged out and therefore reduced.
One known filtering solution for photogrammetry is eliminating by point density. That solution proposes subdividing the terrain under study into a two-dimensional grid corresponding to the ground, e.g. a grid made up of equivalent squares having points with different measured altitudes. The points that present a large amplitude difference from the ground are extracted and form frames that make it possible, after processing with gradient operators, to identify points of discontinuity between portions of the ground and a portion that is detached from the ground. The results obtained in that way are filtered to remove sensor noise or various other forms of interfering noise (see in particular the document by P. Axelsson “Processing of laser scan data—algorithms and applications”, ISPRS Journal of Photogrammetry and Remote Sensing 54 (1999)).
A second field in which filtering out false echoes is important is the field of detecting obstacles. Under such circumstances, the processing of the values required by the telemetry sensor needs to take place as quickly as possible, and the processing must be capable of being performed on a single scan of the scene by the sensor, for example.
An advantageous method of filtering comprises rectifying the image acquired in a ground Cartesian frame of reference with a ground plan that is subdivided into a grid of squares receiving the points together with their specific altitudes. In each square, the point having the highest altitude is identified and the number of points around this maximum altitude in a predetermined range of heights is taken into consideration. For a given square, if the number of points around this maximum altitude point within the range is less than some fixed value, then the maximum altitude point is eliminated. Otherwise this point is retained as the highest point in said square. Thus, a large fraction of points corresponding to noisy measurements in three-dimensional (3D) space are naturally eliminated from each square (see in particular the document by P. W. Lux and C. H. Schaefer “Range imaging for autonomous navigation of robotic land vehicles”, Signal Processing 22, pp. 299-311 (1991)-XP026671937). According to document XP026671937, the altitude of the lowest plot is retained in memory.
The document by K. Zhang et al. “Comparison of three algorithms for filtering airborne lidar data”, Vol. 71, No. 3, March 2005, pp. 313-324-XP002592700 describes filtering with the use of two offset grids and plots obtained from a laser scanner. The idea is to avoid blocking effects at the edge of the grid in geological models such as models of floodplains or flooding.
Document EP 1 243 944 describes measuring the range to a target such as a human body, in which provision is made to remove noise components from the images taken. To obtain images, light irradiation is provided and reflected light is sent together with differences in reflection level as a function of range. Image processing subdivides these into unit images. The unit images are discarded if a signal value exceeds a certain threshold. However the unit images are retained if they are below said threshold.
Document U.S. Pat. No. 5,966,678 descries a system of filtering a range image of a target acquired by laser detection in which the erroneous elements in a range image are detected on the basis of a given criterion and are corrected using data for surrounding range images. That system is complicated and correcting the range image using the surrounding images takes time, which is not appropriate for preparing a local terrain elevation database in real time.
The problem of the present invention is to devise a method and a device enabling a terrain elevation database to be obtained in real time for a zone observed by detector means, said database including all of the portions in relief and the obstacles in said zone, together with a reduced number of errors due to false echoes during data acquisition.
To this end, the invention is defined by the claims.